A dimmer is an electrical apparatus used to change luminous flux and adjust illumination level for a light source in an illumination apparatus. The dimmer is usually connected between a hot and a dimmed hot to adjust light by changing the effective value of the input current of a light source.
The commonly-used dimmer has only two wires to connect with the hot and the dimmed hot, therefore, the dimmer is referred to as a two-wire dimmer. In the two-wire dimmer, a semiconductor power device such as a MOSFET (Metal-Oxide-Semiconductor Field-Effect Transistor), an IGBT (Insulated Gate Bipolar Transistor) or a thyristor may be adopted as a chopper switch. The power network voltage may be controlled by controlling the chopper switch to be on or off. The magnitude of the chopper angle by the chopper switch indicates the level of the light-adjusting signal.
In order to meet requirements of a color lighting system, the two-wire dimmer is required to adjust light in multiple ways such as adjust the color and/or brightness. Generally, there are many circuits in the two-wire dimmer, and even a digital controlling such as a microcontroller is needed. Therefore, an auxiliary power supply for powering the microcontroller or other circuits is needed in the two-wire dimmer.
FIG. 1 is a diagram of an auxiliary power supply circuit of a two-wire dimmer in the prior art. In a case that both chopper switches 110 and 112 are turned off, the auxiliary power supply 124 in the circuit charges the capacitor C1 by using voltage drop generated when the chopper switches are turned off. In the positive half period of the sinusoidal wave, a charging loop is formed by the diode D1, the capacitor C1, the body diode of the chopper switch 112 and the load; in the negative half period of the sinusoidal wave, a charging loop is formed by the diode D2, the capacitor C1, the body diode of the chopper switch 110 and the load. Then an auxiliary voltage Vcc is obtained by voltage regulation of the auxiliary power supply 124.
Since the auxiliary voltage Vcc is obtained by using the voltage generated due to the turn-off of the chopper switches, the minimum chopper angle is needed in order to obtain the minimum auxiliary voltage (the chopper switch is turned off). However, even the chopper switch is off in a very short time (very small angle), a significant electromagnetic interference will be brought into the circuit due to abrupt change of the voltage and the current. In addition, the capacitor C1 may suffer from a peak voltage of the power network voltage, which is converted into a low voltage signal Vcc after passing through the auxiliary power supply 124, to power other circuits, resulting in a high cost and a low efficiency.